Vitreous surfaces may be found on articles of glass, vitrified or enameled articles, articles of quartz, articles of silica and siliceous derivatives, and the like. Treatments to render such surfaces water repellant, or hydrophobic are well known, and are applied, for example, to make bottles drain more completely, to make windshields easier to see through in the rain, to maintain the electrical integrity of porcelain insulators, and the like. However, a dirt deposit resistive feature does not usually accompany such treatments and, in fact, if a silicone coating is used as part of the treatment, the hydrophobic surfaces often attract rather than repel dirt.
For many uses, and particularly in the manufacture of sealed beam type automobile lamps, and exterior window glass, it would be very desirable to provide the outwardly presented vitreous surface with characteristics facilitating the removal of road dirt or air borne dirt. It would also be desirable to provide such characteristics with siliconizing media because of the high efficiency of such media in promoting water repellancy.
Such a treatment should assure reasonable permanency to rubbing and scrubbing or other types of cleaning without loss of the hydrophobic properties of the treated vitreous surfaces. The treatment should also provide a dirt resistive surface, that is, one upon which dirt would not tend to deposit and stick, but even if it should, it would be removed with ease.
The above objectives have been achieved according to the present invention by a unique high temperature interaction of the vitreous surface with a siliconizing medium. The desired reaction takes place considerably above the normal application temperature of similar silicone treatments. The unique results are obtained with vitreous surface temperatures between about 310.degree. and about 370.degree.C. at the time of treatment with the siliconizing medium, and the vitreous surface must be dewatered by heating, e.g., at a temperature of above 400.degree.C. before the treatment. It has been found that if the dewatering step is omitted, the desirable surface characteristics are destroyed during rubbing or similar abrading.
Vitreous surfaces have been siliconized in the prior art under conditions which are superficially similar to those disclosed herein, but in most cases the application temperature is at ambient temperature and the curing temperature is only up to about 250.degree.C. In Kramer et al., U.S. Pat. No. 2,630,656, sheet glass is siliconized at between 38.degree. and 204.degree.C. (100.degree.-400.degree.F.). The glass plates in Hyde, U.S. Pat. No. 2,439,689, are heated at 220.degree.C. for 15 minutes. The polysiloxane coated glass tubes of Goldman, U.S. Pat. No. 2,719,098, are heated in an oven in which the temperature is raised from 100.degree.C. to 250.degree.C. then held for two hours. In Schaefer, U.S. Pat. No. 2,895,846, milk bottles are sprayed with a silicone emulsion and this is bonded thereafter by heating at 149.degree. to 232.degree.C. (300.degree.-450.degree.F.).
On the other hand, the vitreous surface of hot molten glass has been siliconized at temperatures substantially above the range used herein. In one case, Badger, U.S. Pat. No. 2,881,566, used siliconizing at 938.degree. to 1371.degree.C. (1800.degree.F.-2500.degree.F.) to increase the "chemical durability" of the glass.
Two processes for siliconizing glass are known in which the treatment temperature is near to the lower limit of the range discovered in the present invention. The drain-clear containers of Goldman, U.S. Pat. No. 2,504,482 are prepared by coating bottles with a methyl silicone fluid in chloroform and thereafter baking at 300.degree.C. for one-half hour. The vitreous surfaces in U.K. 887,-755 are siliconized by spraying with an organosiloxane onto the glass "while hot", without specifying the glass temperature. However, it is stated that the organopolysiloxane itself is to be applied at a temperature of about 300.degree.C.
One process is known in which the vitreous surface is treated with a siliconizing medium at a treatment temperature overlapping the lower end of the critical range of the present invention. Thus, in Canadian Pat. No. 705,854, vitreous surfaces are treated at 200.degree.-350.degree.C. with a fog of a diorganosiloxane vapor.
The present development provides results which are not obtained from the methods described in U.S. Pat. No. 2,504,482; U.K. No. 887,-755 and Canadian Pat. No. 705,854, however, because of the scrub resistant permanency of the present treatment. As has been mentioned above, unless the vitreous surface is dewatered with a critical heating step prior to the siliconization at 310.degree. to 370.degree.C., the characteristics produced can literally be scrubbed off and the resulting articles are of little commercial value, especially when they are to be exposed outdoors, such as the case with automobile headlamps.
Very few of the prior workers in this field have recognized the value of applying the siliconizing medium to a hot vitreous surface, and of those who did, none appreciated the need to dewater the surface before treatment to assure rub resistance.